Mobile offshore drilling units move from one drill site on the water to another and may be referred to a MODUs. There are two basic types of MODUs used to drill most offshore wells: (1) bottom supported units including submersibles and jack-ups; and (2) floating units including lake barge rigs, drill ships, and semi-submersibles.
Submersible MODUs include swamp barges (sometimes referred to as inland barges) which are used in calm, shallow water environments. A swamp barge comprises a barge hull with drilling rig components mounted thereon. A swamp barge rig is moved from location to location in a floating mode. When the rig reaches a prospective drilling location, a portion of the barge hull is flooded and the barge partially submerges and rests on the bottom of the water body or swamp.
Jack-up MODUs include a large generally triangular-shaped barge hull on which the rig floats when it is being towed from one location to another. The barge hull of a jack-up is commonly referred to in the industry as the platform. Most jack-up rigs have three or four legs which pass through the platform and are connected to a jacking means. When a jack-up is positioned at the prospective well site, the legs are jacked down in contact with the bottom of the water body. When the legs contact the bottom, the platform is jacked up above the wave line. Jack-ups are used in water depths up to about 350 feet.
Lake barge rigs, drill ships, and semi-submersibles are floating units typically used in water depths greater than where a swamp barge or jack-up is applicable. Drill ships are self-propelled and, therefore, incorporate transportation advantages over other MODU rig types which are typically towed from one location to another. Drill ships are best suited for drilling in deep, open waters far removed from shore. A drill ship has a drilling rig mounted in the middle and includes an opening, referred to in the industry as a moon pool, through which drilling operations are conducted. Drill ships are less desirable than semi-submersibles for use in rough water. Semi-submersibles include a lower barge hull which floats below the surface of the sea and is, therefore, not subject to surface wave action. Large stability columns mounted on the lower barge hull support the upper hull, which includes a main deck and machinery deck above the surface of the water.
Lake barges are very similar in construction to swamp barges in that they include a barge on which the drilling rig is mounted. However, in the lake barge rig, the barge element is not intended to be submerged. A lake barge rig is transported in a floating mode but also drills in a floating mode. Lake barges are especially applicable to calm deep water environments. One such environment is Lake Maracaibo in Venezuela. The water is calm but too deep for a swamp barge to rest on bottom. The bottom of the lake is covered in pipelines; therefore, it is not desirable to use a jack-up for fear of piercing one of the pipelines with a leg of the jack-up. Therefore, a floating lake barge is desirable.
Turning now to historical construction techniques for MODUs, several specialized shipyards such as Bethlehem and LeTourneau constructed a substantial portion of the world's fleet of MODUs. Historically, the yards constructed the floating member (hull) first and then added the drilling rig elements, component by component, to the previously constructed hull. The drilling components were not assembled first and then placed as a unit on the hull.
A downturn in the worldwide oil industry in the late 1980's and early 1990's resulted in an oversupply of MODUs. There was virtually no need for construction of new MODUs and many marine shipyards which specialized in building MODUs closed. Those that survived shifted the focus of their construction to other products. The offshore oil industry is now in an upturn and utilization rates for MODUs in some areas have reached over 90 percent.
Presently, most existing shipyards have expertise in building vessels and equipment necessary to construct the floating member (hull) of a MODU. However, such shipyards do not presently have the expertise to efficiently assemble the drilling rig components of the MODU, having shifted the focus of their operations to other products. On the other hand, manufacturers of drilling rig components and contractors who specialize in assembly of land drilling rigs do not have the facilities or expertise to build the hull members of the MODU.
As the currently working rigs age and new rigs are needed to replace them, a demand for a simple, cost effective design and construction techniques for MODUs is desirable. The present invention comprising a mobile offshore drilling unit including a discrete separately fabricated modular subassembly of drilling components (referred to herein as the "rig block") installed on a separately assembled hull meets this need.